Ascorbic acid is used to remove orange rust stains caused by iron in the pool. Large stains are treated by adding products containing ascorbic acid to the surface of the water. Smaller stains are treated by rubbing a tablet containing ascorbic acid back and forth across the stain.
Identifying the type of stain is the first step in treatment. If the stain can't be removed by rubbing a chlorine tablet over it, the stain is probably not organic. Holding a vitamin C tablet against the stain for 30 seconds and observing whether it vanishes or lightens is the best way to confirm if the stain is from iron. Vitamin C tablets contain ascorbic acid.
Though ascorbic acid is effective at removing iron stains from a pool, if the source of the iron is not discovered, the stains will return. Iron is commonly found in well water and less often in municipal water sources. If the iron is from the water source, regular doses of a sequestrant bind to the metals in the water and prevent further staining. Metal objects dropped in the pool generally leave small, concentrated stains that are temporary. Some organic stains resemble metal stains but require a different treatment for removal.